Interferometric imaging of the type IIIb and U radio bursts observed with LOFAR on 22 August 2017

¹ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, R. Prawocheńskiego 9, 10-719 Olsztyn, Poland
e-mail: bartosz.dabrowski@uwm.edu.pl
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, Sofia 1784, Bulgaria
⁴ ASTRON – The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁵ Astronomy & Astrophysics Section, Dublin Institute for Advanced Studies, Dublin 2 D02 XF86, Ireland
⁶ CAS Key Laboratory of Geospace Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China
⁷ Solar–Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, 1180 Brussels, Belgium
⁸ Centre for mathematical Plasma Astrophysics (CmPA), KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
⁹ Department of Physics, University of Helsinki, PO Box 64 Helsinki, Finland
¹⁰ RAL Space, United Kingdom Research and Innovation (UKRI) – Science and Technology Facilities Council (STFC) – Rutherford Appleton Laboratory (RAL), Harwell Campus, Oxfordshire OX11 0QX, UK
¹¹ Astronomical Institute, University of Wrocław, Kopernika 11, 51-622 Wrocław, Poland

Received: 14 December 2021
Accepted: 24 October 2022

Abstract

Context. The Sun is the source of different types of radio bursts that are associated with solar flares, for example. Among the most frequently observed phenomena are type III solar bursts. Their radio images at low frequencies (below 100 MHz) are relatively poorly studied due to the limitations of legacy radio telescopes.

Aims. We study the general characteristics of types IIIb and U with stria structure solar radio bursts in the frequency range of 20–80 MHz, in particular the source size and evolution in different altitudes, as well as the velocity and energy of electron beams responsible for their generation.

Methods. In this work types IIIb and U with stria structure radio bursts are analyzed using data from the LOFAR telescope including dynamic spectra and imaging observations, as well as data taken in the X-ray range (GOES and RHESSI satellites) and in the extreme ultraviolet (SDO satellite).

Results. In this study we determined the source size limited by the actual shape of the contour at particular frequencies of type IIIb and U solar bursts in a relatively wide frequency band from 20 to 80 MHz. Two of the bursts seem to appear at roughly the same place in the studied active region and their source sizes are similar. It is different in the case of another burst, which seems to be related to another part of the magnetic field structure in this active region. The velocities of the electron beams responsible for the generation of the three bursts studied here were also found to be different.